Solid Redox Polymerization Electrodes and Their Use in All-Solid-State Batteries

The dramatic proliferation of products based on semiconductor technology is a strong testament to the predictability and reliability of solid-state technology. In a similar vein, advances in materials exhibiting high levels of ionic conduction in the solid state has led to worldwide efforts in the development of all-solid-state rechargeable batteries. Although higher current densities are achievable with liquid electrolytes, all-solid-state cells offer inherent advantages of negligible self-discharge, long cycle life, high reliability, and virtual absence of container corrosion. Elegant work on polycrystalline and vitreous electrolytes has led to solid electrolytes with suitable ionic conductivities for battery development,1,2,3 however, the superior mechanical properties of elastomeric electrolytes combined with the simplicity of fabrication of thin separator films from these materials, has led to very active pursuit of solid-state cells based on solid polymeric electrolytes (SPE's). Although solid polymer electrolytes generally have lower ionic conductivities than their polycrystalline and vitreous counterparts,4 the ability to cast thin films (10 to 100 μm) from dilute solutions, compensates somewhat for this limitation. While many polymeric electrolytes have been described in the literature, the best known SPE is undoubtedly polyethylene oxide (PEO) complexed with an appropriate electrolyte salt.5 The operating temperature of SPE cells generally falls in the range of ambient to 100°C, with the highest levels of performance occurring in the elevated temperature regime, with claims of performance that meet the demands of electric vehicle application. Initial efforts towards ambient temperature operation of such cells were not promising due to the rapid decrease in ionic conductivity below about 60°C. However, further investigation into the nature of ionic conductivity in polymeric electrolytes has led to the conclusion that ion transport is strictly through the amorphous regions of these polymers which tend to crystallize with decreasing temperature. It has been found that by lowering the glass transition temperature of the polymer electrolyte, either by structural modification or addition of plasticizing electrolyte salts, sufficiently high conductivities for ambient temperature operation can be achieved. Reported results for a number of such cells have indicated that high performance solid-state batteries are nearing commercialization. To date, the majority of these advanced lithium SPE batteries have utilized positive electrodes based on intercalation chemistry. However, work in our laboratory has led to the development of a novel class of solid-state, organic positive electrodes which not only operate on an entirely new principle for energy storage, but which have demonstrated performances exceeding those for intercalation electrodes in solid-state lithium and sodium batteries. These new electrodes have been termed solid redox polymerization electrodes, SRPE's, for reasons that will explained below.